1. Field of the Invention
The present invention relates to a method of preparing an oxide high-temperature superconductor, and more particularly, it relates to a method of preparing an oxide high-temperature superconductor, which can improve the critical current density and is suitable for preparation of a long wire.
2. Description of the Background Art
In recent years, superconductive materials of ceramics, i.e., oxide superconductive materials, are watched as materials which exhibit higher critical temperatures. In particular, a yttrium-based material, a bismuth-based material and a thallium-based material, which exhibit high critical temperatures of about 90.degree. K., 110.degree. K. and 120.degree. K. respectively, are expected as practicable superconductive materials.
It is known that a bismuth superconductor has a phase showing a critical temperature of 110.degree. K. and those showing critical temperatures of 80.degree. K. and 10.degree. K., in particular. It is also known that the 110.degree. K. phase has a 2223 composition in a composition of Bi--Sr--Ca--Cu, or (Bi,Pb)--Sr--Ca--Cu with partial substitution of Pb for Bi, while the 80.degree. K. phase has a 2212 composition of the same elements.
In a general method of preparing an oxide superconductor, raw material for the oxide superconductor is filled into a metal sheath and subjected to deformation processing and heat treatment, so that the raw material contained in the metal sheath is brought into a superconducting state. This method is advantageously applied to preparation of a long superconducting wire, for example.
In order to apply a superconductor to a cable or a magnet, it is necessary to provide a wire, which is formed by such a superconductor, with a high critical current density, in addition to a high critical temperature. Further, such a high critical current density must be homogeneous along the longitudinal direction of the wire.
When the aforementioned method of forming a superconductor in a metal sheath is employed in order to obtain a long wire, however, the superconductor is provided in a state enclosed by the metal sheath and basically cut off from the external atmosphere. Upon heat treatment, therefore, the wire is expanded by gas which is generated from the raw material powder contained in the metal sheath. Thus, the superconductor which is enclosed by the metal sheath may be cracked upon handling during or after the heat treatment etc., to reduce the critical current density of the wire.